RADIANT ENERGY 39 



be applied to the raw materials to convert them into 

 foods. This must obviously come from outside the 

 plant if there is to be any accumulation of food. The 

 energy thus employed reaches the plant in the form of 

 light. 



Radiant Energy. — There reaches the surface of the 

 earth from the sun daily an almost inconceivable amount 

 of energy. Analysis shows that this consists of rays of 

 different wave-lengths, capable of doing different kinds 

 of work. Thus we know that the warmth of the sun's 

 rays raises the temperature of the air and soil and all 

 other objects in their path. Other rays directly affect 

 our organs of vision, and still others are used in making 

 photographs. Sunlight can be divided by a prism as by 

 drops of water into a spectrum (see Fig. 9) or rainbow, 



j Red I Orange] Yellow | Green | Blue | 



2 Leaves 



Fig. 9. — Diagrams to illustrate analysis of light by the spec- 

 troscope, a, spectrum of pure sunlight; 6, spectrum of sunlight 

 passed through chlorophyll. 



made up of seven colors. A thermometer placed beyond 

 the visible color at the red end of the spectrum will show 

 that there are invisible heat or thermal rays in addition 

 to the visible ones which are called light or luminous rays. 

 Similarly a photographic plate exposed beyond the violet 

 end of the spectrum will undergo chemical change, there- 

 by demonstrating that invisible but chemically active 

 rays accompany the invisible heat rays and the visible 

 luminous ones. Thus, we see only a part of the energy 

 which the sun's rays contain, but we know that heat also 

 emanates from the sun, and that certain other invisible 

 rays do chemical work. 

 Photosynthesis. — The combination of carbon dioxid 



